The invention relates to a method and a device for data transfer in a telecommunication system, and particularly to data transfer in a telecommunication system wherein data is transferred cyclically.
An example of a telecommunication system wherein data transfer takes place cyclically in accordance with certain cycle times is a conventional factory automation system comprising a programmable logic controller (PLC or a so-called SoftPLC, which means replacing a programmable logic execution unit by an application program to be run in a computer for the execution of the commands issued by a logic program), related actuators and sensors or other devices, as well as a bus, e.g. a bus called Profibus or the like, connecting these. Typically, the internal telecommunication of such a system is cyclic such that the transmitter of data, e.g. a PLC, transmits a certain (the same) message at certain intervals in accordance with a corresponding cycle time, i.e. in other words a cycle time refers to the time between the starting moments of transmission of two such successive messages. A message is delivered to a device connected to the bus, and the device typically replies by transmitting a reply message. When the cycle time is short enough, data can thus be transferred practically in real time e.g. between a PLC and another actuator, in which case e.g. the real time status information or the like on the actuator is available to the PLC. There are usually several such cycles in progress simultaneously, each having a certain cycle time. The cycle times of different cycles may differ from each other or they may be equal in length. In addition, in such a system a message cycle is usually always started by a device operating as a master, such as a PLC, while the rest of the devices only reply to the messages supplied from the master device. In such a case, each message cycle has a predetermined transfer time on a data transfer path, thus enabling collisions between messages, i.e. simultaneous transmissions to a data transfer path, to be prevented.
Development of data transfer systems and the Internet, for example, have enabled e.g. different remote control systems to be implemented in a simpler manner such that an automation system connected to a general-purpose telecommunication network, such as an Ethernet network, can be monitored and controlled from a control system also connected to the particular network. Furthermore, if the Ethernet network or the like is connected to the Internet, the control system can be connected to the system to be controlled via the Internet.
A problem with the above-described system is how to connect a telecommunication system based on cyclic data transfer to a general-purpose telecommunication system wherein message communication is not based on any regular cycles but takes place as necessary and e.g. in dependence on the load situation in the network. If a message coming outside the system, e.g. via the Internet, is to be delivered to the telecommunication system based on the described cyclic data transfer, and such a message is transferred directly to the system, it is possible that the message may collide with a message of the cyclic communication since no time slot is reserved in the system based on cyclic data transfer for such a message coming from outside. Furthermore, the larger the number of such messages of random character coming from outside a deterministic telecommunication system based on cyclic data transfer and being delivered to the system, the larger the average number of collisions between messages coming from outside and internal cyclic messages of the telecommunication system, in which case the internal data transfer of the telecommunication system becomes increasingly disturbed.
A prior art solution to this problem is disclosed in published application WO 99/13388 which discloses a device for controlling the communication between two telecommunication networks. In the solution disclosed in the publication, a particular intermediation device (proxy device) controls messages supplied to a deterministic network within a certain quota such that it only lets in a certain number of messages during a certain time period. The transfer quota of a device is predetermined according to the load situation of the network.
The problem potentially relating to this prior art solution is, however, that the solution is based on the assumption that by distributing the communication coming from outside a deterministic network evenly within a certain quota, the disturbances caused by the communication coming from outside to the internal communication of the network can be minimized within a certain probability. Particularly when the load of the network is high, collisions may nevertheless occur disturbingly often since the transmission path of the network is highly loaded, meaning that not much free transfer time exists, and the messages differing from the cyclic communication and coming from outside to the transmission path do not necessarily meet the free time slots of the cyclic communication often enough.